A crater lake left behind a diverse mineralogy
In addition to the different ages of the rocks in the vicinity of Jezero Crater, their diverse mineralogical composition is particularly interesting, as researchers can use the minerals to infer the environmental conditions at the time of their formation. The Perseverance rover will be able to study them directly in the crater. The detailed map (topographic image map) of Jezero Crater (top right) shows that the crater rim is breached by three valleys which were former rivers. Neretva Vallis and Sava Vallis were inflow channels that have created two deltas on the western and northwestern rim of the crater, which are also considered evidence for the existence of a former lake. Perseverance will examine the larger of the two in the west in more detail.
Pliva Vallis in the east of Jezero Crater was an outflow channel through which water was discharged from the crater. This is why this former crater lake is referred to as an ‘open basin lake’. These lakes were once numerous on Mars. Compared to closed basins (with inflow but no outflow), they are interesting because they were freshwater lakes with a stable water level. Lakes in closed basins, on the other hand, were subjected to more frequent periods of drying out, which turned them into salt lakes, thus making them less promising in the search for conditions that are conducive to life.
The context map shows the water catchment area of the two inflow channels, from which material from the surrounding area was transported by the rivers into the crater and deposited in the two deltas. Spacecraft orbiting Mars have used spectrometers to detect a variety of minerals in this catchment area. These are mainly silicates from the olivine and pyroxene groups, both mineral classes that originate from magma within the Martian mantle and indicate basaltic volcanic deposits that were not subject to long-term weathering by water. Even more exciting, however, are the carbonates identified on the inner rim of Jezero Crater, which have rarely been found on Mars thus far, and which, together with the clay minerals more frequently found on Mars, testify to the weathering by water of rock with a volcanic source.
Volcanic minerals, carbonates, clay minerals – these three types of deposits have been found both in the delta and elsewhere in the crater. Some carbonates (limestones) are thought to have been formed directly in the lake. Such lake carbonates and especially the clay minerals indicate freshwater conditions that enable life and have the potential to preserve traces of life – biosignatures – particularly well in their interior. However, other types of minerals have also been discovered there, namely those that paint a different picture of the crater lake. These include sulphates that contain iron oxide, amorphous silicon oxides and hydroxides, which tend to form in acidic waters that gradually dried up. These minerals indicate that the environmental conditions in Jezero Crater became drier and less conducive to life at a later stage. However, even among these minerals there are some in which biosignatures can be very well preserved.